A Doppler flow velocity meter may include:
(a) an ultrasonic transducer for the radiation of fluid with ultrasonic wave pulses in response to corresponding transmit pulses which are fed to it with a predetermined pulse repetition rate, for receiving echo signals reflected in the fluid by the particles and for the emission of corresponding echo signals; b) a transmitter linked to the ultrasonic transducer for the production of transmit pulses which excite the ultrasonic transducer to emit ultrasonic wave pulses; (c) a receiver connected to the ultrasonic transducer for receiving and processing echo signals which correspond to at least two different echo waves which are reflected by the particles in a set point of the flow path in response to a first and a second pulse of the transmitted wave, in which adjacent but separate frequency bands of the echo signals are each processed in a separate signal processing channel; (d) an evaluator unit connected to the receiver output, by which an output signal is derived from the Doppler information obtained with the receiver, this signal corresponding to the flow velocity.
By a pulsed Doppler procedure the flow velocity of a fluid, for example, blood carrying particles which reflect ultrasonic waves is measured in an element of determined set volume. In this procedure, a series of ultrasonic pulses are transmitted by a transducer. Each time, at a time .tau. after the transmit pulse, a receiving gate is opened for a brief time for which, EQU .tau.=2 d/c, (1)
in which d is the distance to the volume element and c the sound velocity. Thus a succession of brief reception signals is obtained. These are now processed by suitable electronic means into a continuous Doppler signal. The frequency of this Doppler signal gives the flow point velocity v according to known formula EQU v=-(f.sub.D .multidot.c)/(2 f.sub.1 cos .theta.) (2)
in which f.sub.D is the Doppler frequency, c sound velocity, f.sub.1 the frequency of the ultrasonic signal transmitted and .theta. the angle between the sound radius and the direction of velocity. In this respect one must take care that the Doppler frequency f.sub.D is the difference between the frequency of the signal received and the frequency of the signal transmitted.
Generally a fast Fourier transformation of the Doppler signal is carried out. With this the distribution of flow velocity as a function of time is determined.
In the pulsed Doppler process the echo signal is periodically analysed, that is to say that it is measured only in periodic rest points. According to the Nyquist theorem therefore the maximum univocally measurable Doppler frequency and thus the maximum univocally measurable velocity are limited.
The value of the maximum univocally measurable flow velocity can be determined with the following formula EQU .vertline.v.sub.max .vertline.=(c.sup.2)/(8 f.sub.1 d cos .theta.)(3)
In this formula c is the velocity of sound, f.sub.1 the ultrasonic frequency, d the depth and .theta. the angle between the sonic radius and the direction of the velocity. With this formula one can calculate for example for d=15 cm, .theta.=0, f.sub.1 =3 MHz and c=1540 m/sec and c=1540 m/sec the value v.sub.max =0.66 m/sec. In practice higher velocities are often obtained. These higher velocities are curved ("alias" distortion), which is to say that these are represented as lower velocities or as velocity with an opposite direction. This is a serious disadvantage of this procedure.
In order to widen the measuring range of the point flow velocity, that can be measured univocally with a pulsed Doppler process, the following measures are taken. All these measures however have certain disadvantages.
1. Choice of an emission frequency less than f.sub.1 :
As is shown by equation (3), V.sub.max becomes larger. Consequently however the space resolution and the sensitivity of the procedure are reduced, because at lower frequency less power is redispersed. In practice therefore the lower frequency limit employed is at about 2 MHz.
2. Baseline shift
The normal measuring range lies between -V.sub.max and +V.sub.max. By means of a simple variation of the evaluator it is possible to shift this measuring range, for example, in such a way that it is measured between 0 and 2 v.sub.max. This is however useful only if exclusively positive velocities result. By this means only a shift is obtained and not a broadening of the measuring range.
3. Increase in Pulse Succession Frequency
By means of an increase in the frequency of the pulse series and therefore the analysis rate beyond the normal levels, an increase in the maximum measurable velocity is obtained. The disadvantage of this method is that echo signals from undesired volume elements are also obtained, which are on the outside of the desired volume element to be realised. The undesired volume elements are nearer to the transducer than the desired volume element is and therefore they produce stronger echo signals that disturb the reception of the desired echo signal.
The procedures or measures indicated in points 1 to 3 are all used in practice, which demonstrates that none of these represents a completely satisfactory solution.
Subsequently and also known is a fourth procedure which however is not used in practice.
4. Tracking Doppler Process
In this procedure one determines when the velocity exceeds the Nyquist limits and then a multiple of the velocity V.sub.max is added which is defined by equation (3). The main problem in this procedure is that of reliably determining which Nyquist range one is in. This procedure is not therefore used in practice.
For determining the average value V.sub.a of the flow velocity a so-called two frequency procedure is known which allows an increase of the measuring range with a pulse Doppler method for measuring the flow velocity (see U.S. Pat. No. 4,534,357). In this known two frequency method adjacent but separate echo signal frequency bands are processed in a separate signal processing channel. The highest value V.sub.a max which can be determined by means of this method amounts to: EQU v.sub.a max =(c.sup.2)/(8(f.sub.2 -f.sub.1) d cos .theta.) (4)
where f.sub.1 and f.sub.2 are the average frequencies of the frequency band.
The other symbols are the same as equation (3). Since (f.sub.2 -f.sub.1)&lt;f.sub.0, the maximum measurable velocity V.sub.a max of a two frequency method is greater than the V.sub.max defined by equation (3).
It is important to establish the following difference between the two frequency method just described and the other aforementioned pulsed Doppler procedures described previously:
Using the cited two frequency procedure an average flow velocity value is determined from few measuring points typically from 4 to 8. This method is therefore especially suitable for tracing a colour flow map.
In the other pulsed Doppler methods which have been described above, a spectrum is determined with a relatively large number of measurement points which are typically from 64 up to 256 measuring points. Here therefore substantially more detailed information on the velocity distribution as a function of time is obtained, but more measuring points are also required and thus more time. These methods are therefore utilised for measuring flow velocity in a single volume element.
The object of the present invention is that of providing a Doppler meter of the aforementioned type, by which measurement of the time distribution of flow velocity can be carried out, in which the measurement range where the flow velocity is univocally measured, is notably broadened.
For the solution of this problem a Doppler meter of the aforementioned type according to the present invention is characterised in that (e) the receiver contains the following means:
e.1) means comprised in each of the signal processing channels for the realisation of a demodulation of quadrature of the echo signals received from the ultrasonic transducer in each of the frequency bands, by which at the output of each of the signal processing channels a couple of quadrature signals are obtained each time defining a complex measured value (P(0), P(.DELTA.t), P(2.DELTA.t) . . . );
e.2) Means for the processing of quadrature signals coming from both signal channels, whereby, by these means a first output signal can be obtained which corresponds to an average flow velocity value in the point analysed in the course of the flow;
e.3) Means for the logical correlation of the first output signal with the quadrature signal at the output of one of the signal processing channels, in that, by means of this correlation, signals can be obtained which define complex calculated values of (P(.DELTA.t/2), P(3.DELTA.t/2), . . . ) which are consistent with the average flow velocity determined and with measured measurement values; and
e.4) Means for the evaluation of a signal sequence composed of signals corresponding to complex measured and calculated values (P(0), P(.DELTA.t/2), P(.DELTA.t), P(3.DELTA.t/2), P(2.DELTA.t), . . . ), whereby, by this evaluation, information is obtained on the instantaneous flow velocity value.
The solution according to the present invention with a meter device of the aforementioned kind allows the abovementioned disadvantages of the methods known up to now for increasing the flow velocity measurement range to be eliminated.
In one preferred embodiment the transmitter is set up so that the sequence spectrum of the periodic sequence of the transmit pulses consists of two adjacent but separate frequency bands.
The means for the evaluation of the signal sequence preferably contains means for carrying out analysis of a signal sequence which is composed of signals corresponding to complex measured and calculated values, (P(0), P(.DELTA.t/2), P(.DELTA.t), P(3.DELTA.t/2), P(2.DELTA.t), . . . ) in which by these analyses a second output signal is produced the amplitude of which corresponds to the phase variation of an indicator defined by signals of the signal sequence and therefore corresponds to the instantaneous value of the flow velocity.